STEEL

Iron, like most metals, is not usually found in the Earths crust in an elemental state. Iron can be found in the crust only in combination with oxygen or sulfur. Typical ironcontaining minerals include FeO—the form of iron oxide found as the mineral hematite, and FeS—pyrite fools gold. Iron is extracted from ore by removing the oxygen by combining it with a preferred chemical partner such as carbon. This process, known as smelting, was first applied to metals with lower melting points. Copper melts at just over , while tin melts around . Cast iron—iron alloyed with greater than . carbon—melts at around  . All of these temperatures could be reached with ancient methods that have been used for at least  years since the Bronze Age. Since the oxidation rate itself increases rapidly beyond  it is important that smelting take place in a lowoxygen environment. Unlike copper and tin, liquid iron dissolves carbon quite readily, so that smelting results in an alloy containing too much carbon to be called steel.    Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, with very different properties understanding these is essential to making quality steel. At room temperature, the most stable form of iron is the bodycentered cubic BCC structure ferrite or airon, a fairly soft metallic material that can dissolve only a small concentration of carbon no more than .

Above  °C ferrite undergoes a phase transition from bodycentered cubic to a facecentered cubic FCC structure, called austenite or ?iron, which is similarly soft and metallic but can dissolve considerably more carbon as much as .   As carbonrich austenite cools, the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix,leaving behind iron that is pure enough to take the form of ferrite, resulting in a cementiteferrite mixture. Cementite is a stoichiometric phase with the chemical formula of FeC. Cementite forms in regions of higher carbon content while other areas revert to ferrite around it. Selfreinforcing patterns often emerge during this process, leading to a patterned layering known as pearlite FeC.Fe due to its pearllike appearance, or the similar but less beautiful bainite.Perhaps the most important polymorphic form is martensite, a chemicallymetastable substance with about four to five times the strength of ferrite. A minimum of . wt of carbon CFe is needed to form martensite. When austenite is quenched to form martensite, the carbon is frozen in place when the cell structure changes from FCC to BCC. The carbon atoms are much too large to fit in the interstitial vacancies and thus distort the cell structure into a bodycentered tetragonal BCT structure. Martensite and austenite have an identical chemical composition. As such, it requires extremely little thermal activation energy to form.
                  

                                 PROPERTIES

The heat treatment process for most steels involves heating the alloy until austenite forms, then quenching the hot metal in water or oil, cooling it so rapidly that the transformation to ferrite or pearlite does not have time to take place. The transformation into martensite, by contrast, occurs almost immediately, due to a lower activation energy.Martensite has a lower density than austenite, so that transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, these internal stresses can cause a part to shatter as it cools at the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when water quenched, although they may not always be visible.At this point, if the carbon content is high enough to produce a significant concentration of martensite, the result is an extremely hard but very brittle material. Often, steel undergoes further heat treatment at a lower temperature to destroy some of the martensite by allowing enough time for cementite etc. to form and help settle the internal stresses and defects. This softens the steel, producing a more ductile and fractureresistant metal. Because time is so critical to the end result, this process is known as tempering, which forms tempered steel.

Other materials are often added to the ironcarbon mixture to tailor the resulting properties. Nickel and manganese in steel add to its tensile strength and make austenite more chemically stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while reducing the effects of metal fatigue. Large amounts of chromium and nickel often  and , respectively are added to stainless steel so that a hard oxide forms on the metal surface to inhibit corrosion. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel. On the other hand sulfur, nitrogen, and phosphorus make steel more brittle, so these commonlyfound elements must be removed from the ore during processing.When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. To become steel, it must be melted and reprocessed to remove the correct amount of carbon, at which point other elements can be added. Once this liquid is cast into ingots, it usually must be worked at high temperature to remove any cracks or poorly mixed regions from the solidification process, and to produce shapes such as plate, sheet, wire, etc.


                      HISTORY  OF STEELMAKING


The history of ferrous metallurgy began far back in prehistory, most likely with the use of iron from meteorites. The smelting of iron in bloomeries began in the th century BC in India, Anatolia or the Caucasus. Iron use, in smelting and forging for tools, appeared in SubSaharan Africa by  BC. The use of cast iron was known in the st millennium BC. During the medieval period, means were found in Europe of producing wrought iron from cast iron in this context known as pig iron using finery forges. For all these processes, charcoal was required as fuel.Steel with a smaller carbon content than pig iron but more than wrought iron was first produced in antiquity. New methods of producing it by carburizing bars of iron in the cementation process were devised in the th century AD. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This and other th century and later processes have led to wrought iron no longer being produced.

Because meteorites fall from the sky, some linguists have conjectured that the English word iron OE isern, which has cognates in many northern and Western European languages, derives from the Etruscan aisar which means the gods. Even if this is not the case, the word is likely a loan into preProtoGermanic from Celtic or Italic. Krahe compares Old Irish, Illyrian, Venetic and Messapic forms. The meteoric origin of iron in its first use by humans is also alluded to in the Quran and We sent down Iron in which has incredible strength and many benefits for mankind.Iron was in limited use long before it became possible to smelt it. The first signs of iron use come from Ancient Egypt and Sumer, where around  BC small items, such as the tips of spears and ornaments, were being fashioned from iron recovered from meteorites. However, their use appears to be ceremonial, and iron was probably an expensive metal, perhaps more expensive than gold. About  of meteorites are composed of an ironnickel alloy, and iron recovered from meteorite falls allowed ancient peoples to manufacture small numbers of iron artefacts.

                              ANCIENT STEEL



Steel was known in antiquity, and may have been produced by managing the bloomery so that the bloom contained carbon. Some of the first steel comes from East Africa, dating back to  BC. In the th century BC steel weapons like the Falcata were produced in the Iberian Peninsula, while Noric steel was used by the Roman military. Evidence from ancient Sri Lanka show of steel production as early as  BC  the steel produced was traded in the region and in the islamic world. The Chinese of the Warring States – BC had quenchhardened steel, while Chinese of the Han Dynasty  BC –  AD created steel by melting together wrought iron with cast iron, gaining an ultimate product of a carbonintermediate—steel by the st century AD.Iron appears to have been smelted in the west as early as  BC, but bronze smiths, not being familiar with iron, did not put it to use until much later. In the west, iron began to be used around  BC, presumably as a replacement for bronze, which was becoming harder to come by due to shortages in copper and tin.The onset of the Iron Age in most parts of the world coincides with the first widespread use of the bloomery. While earlier examples of iron are found, their high nickel content indicates that this is meteoric iron. Other early samples of iron may have been produced by accidental introduction of iron ore in bronze smelting operations.

China has long been considered the exception by th century BC, metalworkers in the southern state of Wu had invented the blast furnace, and the means to both cast iron and to decarburize the carbonrich pig iron produced in a blast furnace to a lowcarbon, wrought ironlike material. It was thought that the Chinese skipped the bloomery process completely, starting with the blast furnace and the finery forge to get wrought iron. Recent evidence, however, shows that bloomeries were used earlier in China, migrating in from the west as early as  BC, before being supplanted by the locally developed blast furnace.Early bloomeries were relatively small, smelting less than  kg of iron with each firing. Medieval Europe saw the construction of progressively larger bloomeries, leveling off at around  kg on average, though exceptions did exist. The use of waterwheels to power the bellows allowed the bloomery to become larger and hotter European average bloom sizes quickly rose to  kg, where they leveled off through the demise of the bloomery. Water powered bellows and larger bloomeries also increased the heat to the point where the iron could melt this was not considered desirable because it allowed carbon to diffuse into the molten iron, producing unworkable pig iron. Molten iron was not desirable until the advent of the blast furnace .